This article is about bird taxonomy; for the topic in pottery and glassware design, see Carinate.
The Carinatae are, in phylogenetic taxonomy, the last common ancestor of the Neornithes (modern birds) and Ichthyornis (an extinct seabird of the Cretaceous), and all its descendants. Defined in this way, the group includes all modern birds, both living and recently extinct, and a few Mesozoic forms.
Traditionally, Carinatae were defined as all birds whose sternum (breast bone) has a keel (carina). The keel is a strong median ridge running down the length of the sternum. This is an important area for the attachment of flight muscles. Thus, all flying birds have a pronounced keel. Ratites, all of whom are flightless, lack a strong keel. Thus, living birds were divided into carinates (keeled) and ratites (from ratis, "raft", referring to the flatness of the sternum).
The difficulty with this scheme phylogenetically was that some flightless birds, without strong carinae, are descended directly from ordinary flying birds with carinae. Examples include the Kakapo, a flightless parrot, and the dodo, a columbiform (the pigeon family). None of these birds are ratites. Thus, this supposedly distinctive feature was easy to use, but had nothing to do with actual phylogenic relationship.
The use of a term for keeled sternum to describe the Ichthyornis–Neornithine group turned out to be equally inapt. Various dinosaurs – apparently, remote ancestors and cousins of the Carinatae – do possess a keeled sternum. So, evidently the presence of this structure does not necessarily imply its use in flight. This sort of definitional problem is one reason why the use of physical characteristics to name taxonomic groups is now discouraged.
The characteristics that actually are unique to the Carinatae have little to do with the sternum. Rather, carinates are unique in having, for example, a globe-shaped, convex head on the humerus and fully fused bones in the lower leg and outer arm.
They also have a pterygoid bone that articulates with the palatine by means of a joint. The vomer is reduced or absent.
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